This invention generally relates to processes to form components or parts by etching or partially etching a substrate. In particular, the invention provides a process of selecting sizes and spacings of resist land areas and open areas to control the topography of an etched complex substrate.
Partial depth etching is based on the principles of isotropic etching and the fluid dynamics of etching. Isotropic etching is defined as etching which occurs equally in all directions. As etching of an exposed substrate surface begins, a side wall develops at the boundary or edge of the resist mask land area (area of the substrate that is not being etched or removed by contact with a suitable etchant) and the resist mask open areas (area of the substrate that is being etched or removed by contact with a suitable etchant). Once the formation of the side wall begins at the land area edge, nothing exists to prevent the etching away of the side wall underneath the resist mask land area. This etching of the side wall is commonly referred to as “undercutting”.
The depth of etch divided by the amount of undercut is known as the “etch factor” and describes the shape of an etched recess at a given point in an etch time. Factors influencing the etch depth include such variables as time of etch, spacing distance (width of exposed substrate) between edges or borders of resist mask open areas and land areas, orientation between resist mask openings and resist mask land areas, resist thickness, etchant chemistry and method of etchant application.
One of the important variables in etching is the influence of the spacing distances between edges of the resist mask land areas. Theoretical and experimental studies indicate a strong dependence of the rate of the depth of etch versus the original space width between resist mask edges. Fluid modeling suggests that as the depth of the etched depression forms a cavity, the flow of etchant within the cavity creates one or more flow eddies. This phenomenon causes a reduction in the etch rate, because reactant by-products must traverse these eddies to escape the cavity. This influence increase as the etched cavity deepens.
Studies also indicate that the minimum width of etch openings in a photoresist mask that can be produced is limited or restricted by the widths of the resist mask open areas and the widths of the resist mask land areas. As the widths of the open areas and the land areas are diminished, the amount of undercut will begin to exceed half of the width of the land areas and undercutting from both sides will cause the resist land area to become detached from the substrate. These factors are discussed in Allen et al., Quantitative Examination of Photofabricated Profiles; Part 2—Photoetched Profiles in Stainless Steel, The Journal of Photographic Science, Vol. 26, 1978, pages 72–76.
The etch rate is dependent on several variables. However, the etch rate is particularly dependent on the original width between edges of resist mask openings when the openings have widths of less than about 0.15 mm. For opening widths of less than about 0.15 mm, the etch factor is largest for the widest spacing between edges of resist mask openings, and the etch factor is smallest for the narrowest spacing between edges of resist mask openings. The etch rate is most retarded in narrower spaces between edges of resist mask land areas, such as narrow grooves, slots, small spaces, openings or holes. In such small geometric configurations, spent etchant cannot be easily replenished.
For a particular etchant chemistry and etchant transport method, there exists a critical spacing dimension, referred to as “critical etch space.” When the spacing widths between edges of resist mask land areas are reduced to less than the “critical etch space”, the rate of etching decreases. When the spacing widths between edges of resist mask openings are greater than the “critical etch space”, the rate of etching remains essentially constant. The use of properly sized and spaced resist mask open and land areas has been used to create different thicknesses in substrates. U.S. Pat. No. 5,846,442 reports using appropriately sized and spaced resist mask open and land areas to create two or more areas of different partial etch depths on a single part or substrate without having to use multiple etching stages or steps. This reported process allows fabricating at least two areas of differing thicknesses in the etched substrate using a single etching step.
Although U.S. Pat. No. 5,846,442 generally reports methods of partial etching, it would be beneficial to provide a method of controlling a contour in a transition area of an etched or partial etched complex part or component.